[context] `createContext` function recommendation

The current recommendation for `createContext` has https://github.com/webcomponents-cg/community-protocols/blob/952f15841a809fec73bbaeed842809a2d88b4deb/proposals/context.md?plain=1#L114-L121 This effectively makes `typeof key` to always be `unknown`. [Lit's implementation](https://github.com/lit/lit/blob/bd881370b83d366f7654dd510731242a68949a20/packages/context/src/lib/create-context.ts#L54-L56) has ```ts export function createContext<ValueType, K = unknown>(key: K) { return key as Context<K, ValueType>; } ``` which allows taking an type argument for the context key. In order to take advantage of this though, **both** type arguments must be provided. If only `ValueType` is provided, `K` defaults to `unknown` as TypeScript does not do [partial inference of type arguments](https://github.com/microsoft/TypeScript/issues/26242). Now in practice, I don't think having an explicit `KeyType` be provided to `Context` is quite necessary. The extraction of the `ValueType` does not require it: https://github.com/webcomponents-cg/community-protocols/blob/952f15841a809fec73bbaeed842809a2d88b4deb/proposals/context.md?plain=1#L101-L104 I'm a bit torn as it feels correct to allow explicitly typing the `KeyType` as in the Lit implementation, but if it's functionally moot, that creates confusion like https://github.com/lit/lit/issues/4601 One issue with having `KeyType` be `unknown` is that it can look confusing. With `unknown & {__context: ValueType}`, it just looks like `{__context: ValueType}` in type intellisense. e.g. ```ts const key = Symbol('foo'); const foo = createContext<{foo: string}>(key); // ^? const foo: { // __context__: { // foo: string; // }; // } ```